The invention relates generally to high power, high frequency RF signal manipulation, and in particular to a combiner/splitter for handling high power RF signals.
Switched power combiners for operation at RF frequencies are well known in the art. These units provide the capability of combining the output of several preferably coherent, in phase, signal sources to achieve a higher combined output power. For purposes of this description, it is assumed that all of the signals sources are in phase.
The goal of any design of an N-way power combiner (where N is any positive integer) is to be able to combine the signals on the various input lines, having coherent sources, with little or no penalty due to insertion loss or input VSWR, regardless of how many of the inputs are selected at any given time.
In order to achieve this result, many known configurations have been developed for providing switching along with low insertion loss. Thus, for example, a simple circuit such as that illustrated in FIG. 1 provides a plurality of switched half-wave coaxial transmission lines having a common summing point and connected from that point, through a half-wavelength line to an output. The half-wave-length lines are often implemented as two quarter wave length transmission lines connected in series. Such switchable configurations, however, are typically tuned for a known number of closed switches. As different numbers of switches are closed, the impedance seen at the summing point varies and substantial insertion losses, due to mismatches, can be incurred.
Various techniques have been used to ameliorate this problem so that, for example, different numbers of closed switches will all incur some, but not a lot, of insertion loss.
In one practical application, for example where a cellular base station is provided, it is desirable to use only one signal input source initially and to add additional sources as the volume of calls and users increases. In this situation, the addition of new sources can also require a complete retuning of the combiner, or even its replacement. This is an inconvenience not easily tolerated. Further if a source should fail, its removal could cause a significant power loss, due to mismatching, far beyond the loss of the source itself.
As a result, therefore, it would be desirable to enable hot switching of RF sources without adversely affecting the impedance match of other sources. It is also desirable to provide the ability to vary the number of sources without having to remove or replace the combiner.